The present invention relates to thermal energy management and more particularly relates to management of thermal energy flowing into and out of solid oxide fuel cells.
Fuel cells are electrochemical devices that convert chemical potential energy into usable electricity and heat without combustion as an intermediate step. Fuel cells are similar to batteries in that both produce a DC current by using an electrochemical process. Two electrodes, an anode and a cathode, are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output. Unlike batteries, however, fuel cells do not release energy stored in the cell, running down when battery energy is gone. Instead, they convert the energy typically in a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with fuel and oxidant. Fuel cells emit almost none of the sulfur and nitrogen compounds released by conventional combustion of gasoline or diesel fuel, and can utilize a wide variety of fuels: natural gas, coal-derived gas, landfill gas, biogas, alcohols, gasoline, or diesel fuel oil.
In transportation applications, solid oxide fuel cell (SOFC) power generation systems are expected to provide a higher level of efficiency than conventional power generators, which employ heat engines such as gas turbines, and diesel engines that are subject to Carnot cycle efficiency limits. Therefore, use of SOFC systems as power generators in vehicle applications is expected to contribute to efficient utilization of resources and to a relative decrease in the level of CO2 emissions and an extremely low level of NOx emissions. However, SOFC systems suitable for use in transportation applications require a very compact size as well as efficient thermal management. Thermal management must be accomplished whereby the outer surface of the fuel cell envelope is typically maintained below 45xc2x0 C. while the temperature inside the stack is about 700xc2x0 C. to about 950xc2x0 C. or greater.
As with fuel cells generally, very hot solid oxide fuel cells (SOFC) having high electrical conductivity are used to convert chemical potential energy in reactant gases into electrical energy. In the SOFC, two porous electrodes (anode and cathode) are bonded to an oxide ceramic electrolyte (typically, yttria stabilized zirconia, ZrO2xe2x80x94Y2O3) disposed between them to form a selectively ionic permeable barrier. Molecular reactants cannot pass through the barrier, but oxygen ions (O2xe2x88x92) diffuse through the solid oxide lattice. The electrodes are typically formed of electrically conductive metallic or semiconducting ceramic powders, plates or sheets that are porous to fuel and oxygen molecules. Manifolds are employed to supply fuel (typically hydrogen, carbon monoxide, or simple hydrocarbon) to the anode and oxygen-containing gas to the cathode. The fuel at the anode catalyst/electrolyte interface forms cations that react with oxygen ions diffusing through the solid oxide electrolyte to the anode. The oxygen-containing gas (typically air) supplied to the cathode layer converts oxygen molecules into oxygen ions at the cathode/electrolyte interface. The oxygen ions formed at the cathode diffuse, combining with the cations to generate a usable electric current and a reaction product that must be removed from the cell (i.e., fuel cell waste stream). Typical reactions taking place at the anode (fuel electrode) triple points are:
H2+xc2xdO2xe2x88x92xe2x86x92H2O+2exe2x88x92
CO+xc2xdO2xe2x88x92xe2x86x92CO2+2exe2x88x92
The reaction occurring at the cathode (oxygen electrode) triple points is:
xc2xdO2+2exe2x88x92xe2x86x92O2xe2x88x92
The overall system reactions in the cell are:
xc2xdO2+H2xe2x86x92H2O
xc2xdO2+COxe2x86x92CO2
The consumption of the fuel/oxidant ions produces electrical power where the electromotive force is defined by the Nernst equation:   E  =            E      ⁢              xe2x80x83            ⁢      xc2x0        +                  RT                  2          ⁢          F                    ⁢              ln        ⁡                  (                                    P                              H                2                                      ⁢                                          P                                  O                  2                                                  1                  /                  2                                            /                              P                                                      H                    2                                    ⁢                  O                                                              )                    
Since the SOFC fuel stacks typically operate in such a relatively high temperature range, reactant gases are pre-heated, typically by heat exchangers, to prevent the gases from cooling the stack below the optimum operating temperature. The heat exchangers, whether discrete or part of the total SOFC furnace, can be quite bulky, complex and expensive. In a traditional heat exchanger design, hot exhaust gas from the electrolyte plate is fed to the heat exchanger, and preheated reactant gas is received from the heat exchanger via costly insulated alloy piping. The heat exchanger and piping also require considerable installation and maintenance expense. High temperature piping and heat exchangers are costly from the standpoint of heat loss as well. The piping and heat exchanger have considerable surface area where heat may be exchanged with the atmosphere. This heat is thus unavailable to preheat incoming gases.
It is known to run planar SOFCs having limited numbers of cells in the stack in furnaces. The furnace supplies the heat necessary to bring the SOFC to operating temperature. With a limited number of cells, the surface area of the SOFC stack is large enough to dissipate the energy resulting from the exothermic reaction producing the electric power. In a one or two cell stack, the energy dissipation is such that the stack will require heat input to mitigate losses to the surrounding environment. As the number of cells increases, the effective power density per unit volume increases. The upper limit in SOFC stack size is reached at the point where the stack surface area combined with the maximum acceptable temperature delta at the stack surface is no longer capable of removing the resulting exothermic reaction energy. Acceptable temperature change is limited by the amount of thermally induced stress that the SOFC stack is capable of withstanding. Problematically, this limit is reached well before the power density required for low cost, high volume stack applications is achieved.
One approach to solving this problem is to use the SOFC cathode air to cool or heat the SOFC stack as required by the operating mode. This approach carries with it the fundamental liability of developing thermally induced stress inside the SOFC stack unless the air temperature has a limited temperature delta from the operating point of the stack. However, such a limit restricts efficiency by requiring a high volume of air to remove the thermal energy. In fact, one of the largest parasitic loads in a SOFC system is the blower used to supply the cathode air.
What is needed in the art is an improved SOFC thermal management system. What is further needed in the art is a compact, efficient SOFC thermal management system suitable for transportation applications.
The present thermal energy management system and method for controlling the thermal energy flow into and out of a SOFC comprises a monolithic ceramic heat exchanger coupled to a SOFC stack. The heat exchanger controls the energy flow into and out of the SOFC stack and manages the thermal energy produced as a byproduct of the operation of the SOFC stack. The system further provides management of the temperature distribution around the SOFC to meet outer skin temperature design targets and to control the inlet gas temperatures for the SOFC.
The heat exchanger comprises a small-cell co-extrusion monolithic type heat exchanger similar to a catalytic converter core. The heat exchanger includes an air inlet side, an air outlet side, and a plurality of cells for passing a flow of air therethrough; the heat exchanger being coupled to a SOFC stack. In operation, a flow of inlet air having a selected temperature is passed through the heat exchanger cells and thermal energy flowing into and out of the SOFC stack is managed primarily by radiation coupling between the SOFC stack and the heat exchanger.
The material used to prepare the present heat exchanger may be selected in accordance with the system requirements, with suitable materials including, but not being limited to, ceramics, zirconium phosphate, silicon nitride, aluminum nitride, molybdenum disilicide, zirconia toughened aluminum oxide, aluminum phosphate, zirconium oxide, titanium carbide, aluminum oxide, zirconium carbide, zirconium disilicide, alumino-silicates, cordierite and silicon carbide.
The present heat exchanger is operational over the entire range of temperatures generated by the SOFC (i.e., temperatures up to about 1000xc2x0 C.). By using a ceramic structure as an xe2x80x9cair-to-airxe2x80x9d heat exchanger, several advantages are provided simultaneously. First, the present invention advantageously provides a cool skin temperature using minimum space. The present invention reduces the temperature from about 1000xc2x0 C. to about 100xc2x0 C. in an area of about 6 to about 10 millimeters.
Second, the present system advantageously controls the amount of heat energy removed from the cells based on the electrical power demand. This allows the cooling airflow to be modulated at low temperature using low cost hardware and further used to control the high temperature heat flow from the SOFC.
Third, the present heat exchanger functions as a pre-heater for the input fuel and oxidizer gases feeding the SOFC stack. Thus, the present system advantageously controls the temperature of the input fuel and oxidizer gases prior to their introduction into the SOFC.